Wireless strain measurement attracts attention of many researchers and finds application in numerous fields when it comes to material characterization. In civil engineering, the sensors of the sort are used to ensure maintenance in infrastructure, by measuring the strain on the structure and take precaution in case of any abnormal strain (T. Nagayama, M. Ruiz-Sandoval, B. F. Spencer Jr., K. A. Mechitov, G. Agha, “Wireless Strain Sensor Development for Civil Infrastructure”, Trans. of the Society of Instrument and Control Engineers, Vol. E-3, No. 1, 104/109 (2004)). In biomedical engineering, these sensors are utilized by observation of the healing process of a fractured bone (R. Melik, N. K. Pekgoz, E. Unal, C. Puttlitz, and H. V. Demir, “Bio Implantable passive on-chip RF-MEMS strain sensing resonators for orthopaedic applications,” J. Micromech. Microeng. 18(11), 115017 (2008)). Moreover, in aerospace industry, wireless strain measurement comes into prominence in crack or abnormal strain detection on metal surfaces (Rajni, A. Kaur, A. Marwaha, “Crack Detection on Metal Surfaces with an Array of Complementary Split Ring Resonators”, International Journal of Computer Applications (0975-8887) Volume 119-No. 10, June 2015). SRR-based sensors have been demonstrated for wireless strain measurement previously. The resonant frequency of the device strongly depends on the geometry of the ring and any deviation in geometry due to external strain applied to the device results in change in resonant frequency of the resonator. This enables strain measurement through the observation of the shift in resonant frequency.
Glaucoma is an eye disease, which may damage the optic nerves and leads to vision loss. Even though it might be caused by different factors, in most of the patients, it is caused by the increase of intraocular pressure and might eventually cause irreversible blindness. Today, there are several different methods to detect the symptoms of glaucoma, such as measuring central corneal thickness, measuring peripheral vision, examining the optic nerve and measuring the eyeball pressure (“Five Common Glaucoma Tests”, glaucoma.org, Glaucoma Research Foundation, Apr. 22, 2013). It was demonstrated that the radius of curvature of the sclera is well correlated with the intraocular pressure, while the radius of the curvature of the cornea is insensitive to the changes in intraocular pressure (B. K. Pierscionek, M. Asejczyk-Widlicka, R. A. Schachar, “The effect of changing incraocular pressure on the corneal and scleral curvatures in the fresh porcine eye”, Br J. Ophthalmol 2007; 91:801-803. doi: 10.1136/bjo.2006.110221). Noninvasive monitoring of the intraocular pressure has been demonstrated using piezoresistive (M. Leonardi, P. Leuenberger, D. Bertrand, A. Bertsch, P. Renaud, “First Steps toward Noninvasive intraocular Pressure Monitoring with a Sensing Contact Lens”, Invest. Ophthalmol. Vis. Sci., 45(9), 3113 (2004)) and capacitive (D. Piso, P. Veiga-Crespo, E. Vecino, “Modern Monitoring Intraocular Pressure Sensing Devices Based on Application Specific Integrated Circuits”, Journal of Biomaterials and Nanobiotechnology, 2012, 3, 301-309) sensors embedded on soft contact lenses. It is possible to monitor the progress of the disease in noninvasive and continuous manner using this approach. However, these sensors are electrically active and require application of electrical signals in the contact lens during operation. In addition, the system on the contact lens also includes a transmission circuitry to send the signals to an external unit.
Conventional methods of glaucoma detection using contact lenses employ electrically active sensors and readout electronics embedded with the lenses (M. Leonardi, P. Leuenberger, D. Bertrand, A. Bertsch, P. Renaud, “First Steps toward Noninvasive Intraocular Pressure Monitoring with a Sensing Contact Lens”, Invest. Ophthalmol. Vis. Sci., 45(9), 3113 (2004)). The sensors should be powered up electrically to operate and the processed data should be transferred using wireless RF links. This poses a significant limitation for the systems to be integrated on contact lenses that are in continuous contact with eye. First, it is not desirable to have electrically active circuits in contact with eye. Secondly, the power requirement for these systems can be demanding. In addition, both packaging and the implementation of the sensor with integrated readout electronics are expensive for a disposable sensor.